The amount of desiccant used is a crucial factor in the application of desiccants. If too little desiccant is used, it will not effectively prevent moisture and the product will be affected by moisture; If too much desiccant is used, it will result in waste and be uneconomical.
1. Setting the target humidity inside the product packaging
Before starting to calculate the amount of desiccant, we must set a target humidity, which is the maximum allowable humidity inside the packaging. On one hand, ordinary products can be safely stored in environments with humidity below 50%, but there are also many products that require lower humidity levels, such as 20%. The lower the target humidity setting, the more desiccants need to be used to keep the packaging dry.
2. Moisture transmission rate of barrier packaging
The water vapor permeability (g/m2 · day) refers to the weight in grams of water vapor that penetrates the test material after 24 hours (one day) of testing in a static environment under stable temperature and humidity conditions.
The water vapor transmission rate of barrier packaging has a significant impact on the amount of desiccant used. The same volume of packaging requires the same amount of storage time. If materials with good barrier properties, such as aluminum foil, are used, less desiccant can be used. If materials with poor barrier properties, such as thin PE bags, need to be filled with a lot of desiccant to achieve the same effect.
So, if conditions permit, we recommend using barrier materials with good barrier properties (i.e. low water vapor permeability) to package products as much as possible. In addition, the sealing of the barrier packaging must be tight and absolutely not have any air holes. Any small air leakage hole will greatly reduce the effectiveness of the desiccant.
The water vapor permeability of the same barrier material varies under different temperature and humidity conditions. The higher the temperature and humidity, the higher the water vapor transmission rate. But when we calculate, we can only take an average value to use. DIN55474 suggests that we use the value of water vapor permeability under conditions of 23 ℃ and 85% RH. If the value is taken in combination with the actual situation, it will make the calculation more accurate.
3. Three sources of moisture in packaging
In a specific packaging, there are three sources of moisture, and the mission of desiccants is to absorb these three parts of water vapor and control the humidity inside the packaging below the target humidity.
(1) The initial moisture contained in the air inside the product packaging. For example, the volume of product packaging is 1 cubic meter, and the initial temperature and humidity during packaging are 23 ℃. When the relative humidity is 85%, the air contains 17.47 grams of water.
(2) The moisture contained in the packaging materials inside the product packaging will gradually evaporate during storage and transportation. For example, wood usually has a moisture content of 15-30%.
(3) During storage and transportation, water vapor that penetrates into the product packaging is blocked by the packaging material. For example, a barrier PE bag with a water vapor transmission rate of 2 grams per square meter per day and an area of 6 square meters will permeate 1080 grams of water into the packaging after 90 days
4. Calculation formula and parameters for desiccant dosage
n =1/a*( V *b+ m * c + A *e* WDD * t )
n : Number of units of desiccant required
a : The amount of water vapor that a unit of desiccant can absorb under target humidity conditions (in grams per gram)
V: Internal volume of packaging (cubic meters m3)
b : Air water content under temperature and humidity conditions during initial packaging (g/cubic meter g/m3)
